In two-stage evaporation installations for the purification of radio-active liquid wastes, vapor arising from the evaporation of the liquid in the first stage flows into a mixing condenser disposed intermediate the first and second stages. In such mixing condenser, the vapor in the first conduit is condensed by contact with liquid introduced into the mixing condenser via a feed back path from the second stage. Such feed back path includes a circulating pump that maintains a back pressure in the mixing condenser that corresponds to the normal pressure difference between the first and second stages arising from the higher vapor temperature in the first stage.
During a normal work cycle of such installation wherein the pump is in operation, vapor flow is propelled from the higher-pressure first stage into the mixing condenser via the first conduit. However, during start-up of the installation when the pump is first placed into operation with the mixing condenser filled to its normal working level, the absence of vapor pressure in the first evaporation stage would normally tend to cause a backflow of liquid from the mixing condenser into such first stage, since the first conduit represents a path of least resistance for such backflow.
Several arrangments have been proposed in the past for preventing such backflow. In one arrangement, a regulation device responding to the pressure in the first stage controls continuously the pressure produced by the circulation pump of the second stage to an equal value of the first stage. Another proposal suggests that the first conduit should be arranged vertically to such a height that the pressure produced by the circulation pump of the second stage could be balanced by a water column in this vertical conduit in case of absence of the pressure of the first stage. A third arrangement employs a blocking member interposed in the first conduit and adapted to be opened only when the required pressure differential conditions are established in the installation.
All of such arrangements are technically complex and/or expensive.